Process for preparing lower olefins from an oxygenate

ABSTRACT

The invention relates to a process for preparing lower olefins from an oxygenate, the process comprising subjecting C4 hydrocarbons obtained in an oxygenate-to-olefins conversion step to extractive distillation to obtain a stream enriched in unsaturated C4 hydrocarbons comprising isobutene and n-butenes, and a stream enriched in saturated C4 hydrocarbons; converting the isobutene in the stream enriched in unsaturated C4 hydrocarbons into an alkyl tertiary butyl ether to obtain an isobutene-depleted unsaturated C4 hydrocarbon stream and alkyl tertiary-butyl ether; and recycling at least part of the isobutene-depleted unsaturated C4 hydrocarbon stream and/or at least part of the alkyl tertiary-butyl ether, optionally after conversion into tertiary butanol and/or isobutene, to the oxygenate-to-olefins conversion step.

This application claims the benefit of European Patent Application No.11195850.0, filed Dec. 28, 2011, the entire disclosure of which ishereby incorporated by reference.

FIELD OF THE INVENTION

The invention relates to a process for preparing lower olefins from anoxygenate.

BACKGROUND TO THE INVENTION

Conventionally, lower olefins such as ethylene and propylene areproduced via steam cracking of hydrocarbon feedstocks including ethane,propane, naphtha, gasoil and hydrowax. An alternative route to lowerolefins is the so-called oxygenate-to-olefin process. In suchoxygenate-to-olefin process, an oxygenate such as methanol ordimethylether (DME) is provided to a reaction zone containing a suitableoxygenate conversion catalyst, typically a molecular sieve-comprisingcatalyst, and converted into ethylene and propylene. In addition to thedesired lower olefins, a substantial part of the oxygenate is convertedinto C4+ olefins and paraffins.

In WO2009/065848 is disclosed an oxygenate-to-olefin process wherein theyield of lower olefins is increased by recycling a fraction comprisingC4+ olefins to the reaction zone. At least part of the C4+ olefins inthe recycle are converted into the desired lower olefins. A disadvantageof the process of WO2009/065848 is, however, that at least part of therecycle stream needs to be purged in order to avoid undesiredaccumulation of paraffins in the recycle stream. With the purge, alsovaluable C4+ olefins will be removed from the process without beingconverted into lower olefins.

In U.S. Pat. No. 7,923,591 is disclosed a process for manufacturinglower olefins from an oxygenate-containing reaction mixture, wherein aproduct stream comprising C4 and C5 hydrocarbons from the oxygenateconversion step is subjected to extractive distillation to separatesaturated butanes from it. The remaining butenes and C5 hydrocarbonsare, after solvent stripping, recycled to the oxygenate conversion step.

SUMMARY OF THE INVENTION

It has now been found that by subjecting a fraction comprising C4hydrocarbons from the effluent of an oxygenate-to-olefin step toextractive distillation for separation into saturated and unsaturated C4hydrocarbons and then subjecting the unsaturated C4 hydrocarbons to anetherification step for conversion of the isobutene into an alkyltertiary butyl ether, it is possible to recycle butenes to theoxygenate-to-olefin step without undesired accumulation of paraffins.Moreover, by subjecting the butenes separated by the extractivedistillation step to an etherification step that selectively convertsisobutene, a separation between normal and isobutene has been effected.Thus, the ratio of normal and isobutene to be recycled to theoxygenate-to-olefin step can be controlled. Isobutene may be recycled inthe form of the alkyl tertiary butyl ether produced in theetherification step, in the form of a tertiary butanol produced from thealkyl tertiary butyl ether, or in the form of isobutene produced fromsuch tertiary butanol or such alkyl tertiary butyl ether. Under thereaction condition prevailing in an oxygenate-to-olefin step, tertiarybutanol or alkyl tertiary butyl ether will typically be converted intoisobutene.

Accordingly, the present invention relates to a process for preparinglower olefins from an oxygenate, the process comprising the followingsteps:

-   a) contacting the oxygenate with a molecular sieve-comprising    catalyst, at a temperature in the range of from 350 to 1000° C. to    obtain an olefinic product stream comprising ethylene, propylene and    C4 hydrocarbons;-   b) separating ethylene and/or propylene and a fraction comprising C4    hydrocarbons including saturated and unsaturated C4 hydrocarbons,    from the olefinic product stream;-   c) subjecting at least part of the fraction comprising C4    hydrocarbons to extractive distillation to obtain a stream enriched    in unsaturated C4 hydrocarbons comprising isobutene and n-butenes,    and a stream enriched in saturated C4 hydrocarbons;-   d) supplying at least part of the stream enriched in unsaturated C4    hydrocarbons obtained in step c) and an alcohol selected from the    group consisting of methanol, ethanol and a mixture thereof, to an    etherification reaction zone comprising an etherification catalyst    and reacting, in the etherification reaction zone, at least part of    the isobutene in the stream enriched in unsaturated C4 hydrocarbons    with the alcohol to obtain an etherification product stream    comprising alkyl tertiary butyl ether;-   e) separating the etherification product stream into an alkyl    tertiary butyl ether-enriched stream and an isobutene-depleted    unsaturated C4 hydrocarbon stream; and-   f) recycling at least part of the isobutene-depleted unsaturated C4    hydrocarbon stream and/or at least part of the alkyl tertiary-butyl    ether to step a).

The stream enriched in saturated C4 hydrocarbons obtained in step c) canadvantageously be supplied to for example a steam cracking process, anLPG bleeding pool or to an isomerisation process for conversion intoisobutane.

It has the further advantage that a normal butene product, depleted iniso-olefins and paraffins may be retrieved as a product stream. This cansuitably be used for instance an alkylation process. Alternatively, oreven additionally, it is possible to retrieve a MTBE product stream.Such a stream may suitably be used in a process for producing propyleneoxide.

BRIEF DESCRIPTION OF THE DRAWING

The FIGURE schematically shows a process according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

In the process according to the invention, an oxygenate is firstconverted into lower olefins by contacting the oxygenate with amolecular sieve-comprising catalyst at a temperature in the range offrom 350 to 1000° C. (oxygenate conversion step a)). Besides lowerolefins, i.e. ethylene and propylene, C4 olefinic and paraffinichydrocarbons and, in a lesser amount C5+ olefinic and paraffinichydrocarbons are formed as by-product. Thus, an olefinic product streamcomprising ethylene, propylene, C4 hydrocarbons and higher hydrocarbonsis obtained in step a). Typically, C4+ paraffins such as isobutane,n-butane, n-pentane, iso-pentane, and C4+ olefins such as isobutene,n-butenes, n-pentenes, iso-pentenes and C5+ naphtenes such ascyclopentane and cyclopentene will be present in the olefinic productstream. Small amounts of dienes like butadienes may be present in theolefinic product stream.

Reference herein to an oxygenate is to a compound comprising at leastone alkyl group that is covalently linked to an oxygen atom. Preferably,at least one alkyl group has up to five carbon atoms, more preferably upto four, even more preferably one or two carbon atoms, most preferablyat least one alkyl group is methyl. Mono-alcohols and dialkylethers areparticularly suitable oxygenates. Methanol and dimethylether or mixturesthereof are examples of particularly preferred oxygenates.

The oxygenate conversion in step a) is carried out by contacting theoxygenate with a molecular sieve-comprising catalyst at a temperature inthe range of from 350 to 1000° C., preferably of from 350 to 750° C.,more preferably of from 450 to 700° C., even more preferably of from 500to 650° C. The conversion may be carried out at any suitable pressure,preferably at a pressure in the range of from 1 bar to 50 bar(absolute), more preferably of from 1 bar to 15 bar (absolute). Apressure in the range of from 1.5 to 4.0 bar (absolute) is particularlypreferred.

Any molecular sieve comprising catalyst known to be suitable for theconversion of oxygenates, in particular alkanols and dialkylethers, intolower olefins may be used. Preferably the catalyst comprises a molecularsieve having a 8-, 10- or 12-ring structure and an average pore size inthe range of from 3 Å to 15 Å. Examples of suitable molecular sieves aresilicoaluminophosphates (SAPOs), aluminophosphates (AlPO),metal-substituted aluminophosphates or metal-substitutedsilicoaluminophosphates. Preferred SAPOs include SAPO-5, -8, -11, -17,-18, -20, -31, -34, -35, -36, -37, -40, -41, -42, -44, -47 and -56.SAPO-17, -18, -34, -35, and -44 are particularly preferred.

A particular suitable class of molecular sieves are zeolites. Inparticular in case not only oxygenates but also C4+ olefins or compoundsthat form C4+ olefins under the reaction conditions prevailing inoxygenate conversion step a) are supplied to step a), e.g. a tertiaryalkylether such as methyl tertiary butylether, a zeolite-comprisingcatalyst is preferred as molecular-sieve comprising catalyst, morepreferably a catalyst comprising a zeolite with at least a 10-memberedring structure. Zeolite-comprising catalysts are known for their abilityto convert higher olefins to lower olefins, in particular C4+ olefins toethylene and/or propylene. Suitable zeolite-comprising catalysts includethose containing a zeolite of the ZSM group, in particular of the MFItype, such as ZSM-5, the MTT type, such as ZSM-23, the TON type, such asZSM-22, the MEL type, such as ZSM-11, the FER type. Other suitablezeolites are for example zeolites of the STF-type, such as SSZ-35, theSFF type, such as SSZ-44 and the EU-2 type, such as ZSM-48. Preferably,the catalyst comprises at least one zeolite selected from MFI, MEL, TONand MTT type zeolites, more preferably at least one of ZSM-5, ZSM-11,ZSM-22 and ZSM-23 zeolites.

The zeolite in the oxygenate conversion catalyst is preferablypredominantly in the hydrogen form. Preferably at least 50 wt %, morepreferably at least 80 wt %, even more preferably at least 95 wt %,still more preferably at least 100 wt % of the zeolite is in thehydrogen form.

The molecular sieve-comprising catalyst may further comprise a bindermaterial such as for example silica, alumina, silica-alumina, titania,or zirconia, a matrix material such as for example a clay, and/or afiller.

The present molecular sieve catalyst may comprise phosphorus as such orin a compound, i.e. phosphorous other than any phosphorus included inthe framework of the molecular sieve. It is preferred that an MEL orMFI-type zeolite comprising catalyst additionally comprises phosphorus.The phosphorus may be introduced by pre-treating the MEL or MFI-typezeolites prior to formulating the catalyst and/or by post-treating theformulated catalyst comprising the MEL or MFI-type zeolites. Preferably,the present molecular sieve catalyst comprising MEL or MFI-type zeolitescomprises phosphorus as such or in a compound in an elemental amount offrom 0.05-10 wt % based on the weight of the formulated catalyst. Aparticularly preferred catalyst comprises phosphorus and MEL or MFI-typezeolites having SAR of in the range of from 60 to 150, more preferablyof from 80 to 100. An even more particularly preferred catalystcomprises phosphorus and ZSM-5 having SAR of in the range of from 60 to150, more preferably of from 80 to 100.

In step a), not only lower olefins and C4+ hydrocarbons, but also wateris formed. Water is typically separated from the olefinic product streamby means known in the art, for example by cooling the effluent of stepa) in a water quench tower.

In step b) of the process according to the invention, ethylene and/orpropylene and a fraction comprising C4 hydrocarbons are separated fromthe olefinic product stream obtained in step a). Such separation indifferent fractions is done by means known in the art. Typically, thestream is fractionated in at least a fraction mainly comprising ethyleneand/or propylene and a fraction comprising C4 hydrocarbons. Usually, afraction comprising mainly ethylene is first separated from the olefinicproduct stream in a de-ethaniser and a fraction mainly comprisingpropylene is then separated from the bottoms of the de-ethaniser in ade-propaniser. Instead of fractionating the olefinic product stream intoseparate ethylene and propylene fractions, a fraction comprising bothethylene and propylene may be obtained by directly supplying theolefinic product stream to a de-propaniser. The bottoms of thede-propaniser contain C4+ hydrocarbons. Preferably, the bottoms of thede-propaniser is further separated into a fraction mainly comprising C4hydrocarbons and a fraction comprising C5+ hydrocarbons. The fractioncomprising C4 hydrocarbons comprises saturated and unsaturated C4hydrocarbons. The unsaturated C4 hydrocarbons comprise isobutene andn-butenes.

In step c) at least part of the fraction comprising C4 hydrocarbonsobtained in step b) is subjected to extractive distillation in order toremove saturated C4 hydrocarbons, i.e. butanes, from the C4 olefins inthat fraction. Thus, a stream enriched in unsaturated C4 hydrocarbonscomprising isobutene and n-butenes, and a stream enriched in saturatedC4 hydrocarbons comprising normal and iso-butane, are obtained.Butadiene, if present, may be removed from the fraction comprising C4hydrocarbons prior to subjecting the fraction to extractive distillationfor removal of saturated C4 hydrocarbons.

Part of the fraction comprising C4 hydrocarbons obtained in step b) maybe recycled to step a).

In step c), at least part of the fraction comprising C4 hydrocarbons is,optionally after removal of butadiene, subjected to extractivedistillation. In extractive distillation step c), a stream enriched inunsaturated C4 hydrocarbons and a stream enriched in saturated C4hydrocarbons are obtained. The extractive distillation is carried out bysupplying at least part of the fraction comprising C4 hydrocarbons to anextractive distillation column together with a suitable solvent, such asfor example dimethylformamide (DMF), N-formylmorpholine (NFM),acetonitrile or N-methylpyrrolidone. Saturated C4 hydrocarbons leave thecolumn over the top and solvent containing unsaturated C4 hydrocarbonsleaves the column via the bottom. In case the fraction comprising C4hydrocarbons comprises C5+ hydrocarbons, the bottom fraction willfurther comprises such C5+ hydrocarbons.

Separation of butanes from butenes by means of extractive distillationis well-known in the art. Any suitable process conditions and solventsknown in the art may be applied.

Solvent is separated from the butenes by means known in the art,typically by stripping, in order to obtain the stream enriched inunsaturated C4 hydrocarbons.

The prior removal of butadiene is typically done by extractivedistillation. Thus, in case butadiene is removed prior to butane/butaneseparation, the process typically comprises two extractive distillationsteps: a first extractive distillation step to remove butadiene and asecond extractive distillation step to separate butenes from butanes. Inthe first step, the fraction comprising C4 hydrocarbons and a suitablesolvent are supplied to a first extractive distillation column. A streamcomprising butenes and butanes will leave the column over the top andsolvent containing butadiene leaves the column via the bottom. In thesecond step, the stream comprising butenes and butanes is suppliedtogether with a suitable solvent to a second extractive distillationcolumn. The saturated C4 hydrocarbons (butanes) leave the column overthe top and solvent containing unsaturated C4 hydrocarbons includingisobutene and n-butenes leaves the column via the bottom. In each of theextractive distillation steps, any suitable process conditions andsolvents known in the art may be applied. Preferably, the same solventis used in both extractive distillation steps.

The stream enriched in saturated C4 hydrocarbons (top stream of theextractive distillation column) may be withdrawn from the process andfor example used for steam cracking, blended into an LPG pool, orisomerised to isobutane.

In step d), at least part of the stream enriched in unsaturated C4hydrocarbons is supplied to an etherification reaction zone togetherwith an alcohol. The etherification zone comprises an etherificationcatalyst and in this zone at least part of the isobutene in the streamenriched in unsaturated C4 hydrocarbons is reacted with the alcohol tobe converted into alkyl tertiary butyl ether. In case the streamenriched in unsaturated C4 hydrocarbons also comprises iso-pentenes,also alkyl tertiary amyl ether is formed. Thus, an etherificationproduct stream comprising alkyl tertiary butyl ether is obtained. Thealcohol is selected from the group consisting of methanol, ethanol and amixture thereof. Preferably, the alcohol is methanol and anetherification product stream comprising methyl tertiary butyl ether isobtained. Etherification of isobutene to form an alkyl tertiary butylether is well-known in the art. Any catalyst and process conditionsknown to be suitable for such etherification may be used. Typically, theetherification catalyst is an acid catalyst. Preferably, theetherification catalyst is a protonated cation-exchange resin or aheteropolyacid promoted by a metal. A particularly preferred catalyst isAmberlyst-15. Preferably, the etherification reaction is carried out ata temperature in the range of from 40 to 100° C., more preferably offrom 50 to 85° C. The reaction may be carried out at any suitablepressure, preferably in the range of from 1 to 20 bar (absolute), morepreferably of from 5 to 15 bar (absolute).

In a subsequent step e), the etherification product stream is separatedinto an alkyl tertiary butyl ether-enriched stream and anisobutene-depleted unsaturated C4 hydrocarbon stream. This may be doneby any suitable means known in the art, for example by distillation.

In step f), at least part of the isobutene-depleted unsaturated C4hydrocarbon stream and/or at least part of the alkyl tertiary-butylether is recycled to oxygenate-to-olefin conversion step a).

Preferably at least part of the isobutene-depleted unsaturated C4hydrocarbon stream is recycled to step a), more preferably at least 50wt %, even more preferably at least 90 wt %. Since theisobutene-depleted unsaturated C4 hydrocarbon stream does not or hardlycomprise saturated hydrocarbons, the entire isobutene-depletedunsaturated C4 hydrocarbon stream may be recycled to step a). Recyclingof a stream depleted in isobutene is particularly advantageous in case amolecular sieve comprising catalyst is used in step a) that does not ordoes hardly catalyse the conversion of isobutene into lower olefins.Examples of such catalysts are SAPO-containing catalysts, in particulara SAPO-34 containing catalyst.

Alternatively, or in combination with a recycle of at least part of theisobutene-depleted unsaturated C4 hydrocarbon stream, at least part ofthe alkyl tertiary-butyl ether produced in step c) is recycled tooxygenate-to-olefin conversion step a). Such recycle of alkyltertiary-butyl ether is advantageous in case a molecular sievecomprising catalyst is used in step a) that is able to catalyse theconversion of isobutene into lower olefins, as is typically the case fora zeolite-comprising catalyst, in particular a catalyst comprising azeolite with a 10-membered ring structure. Alkyl tertiary-butyl ethermay be recycled as such to step a) or in the form of tertiary butanoland/or isobutene, i.e. after conversion into tertiary butanol and/orisobutene.

The alkyl tertiary-butyl ether may be decomposed to back to the alcoholand the iso-olefins, or optionally the alcohol and an iso-paraffin. Inthat case the methanol may be recycled to the etherification process.

The alkyl tertiary butyl ether formed in step d) can advantageously beconverted into tertiary butyl hydroperoxide, which can be converted intoan epoxide by reacting it with ethylene and/or propylene separated fromthe olefinic product stream obtained in step a). Thus, an integratedprocess for preparing an epoxide from an oxygenate is provided. Suchprocess further comprises: converting at least part of the alkyltertiary butyl ether in the alkyl tertiary butyl ether-enriched streaminto the alcohol and isobutane (step g)); oxidizing isobutane obtainedin step g) into tertiary butyl hydroperoxide (step h)); and reacting thetertiary butyl hydroperoxide with ethylene and/or propylene separatedfrom the olefinic product stream obtained in step a) to obtain theepoxide and tertiary butanol (step i)).

At least part of the alkyl tertiary butyl ether in the alkyl tertiarybutyl ether-enriched stream may be converted into the alcohol andisobutane, for example by first cracking alkyl tertiary butyl ether intoisobutene and the alcohol and then hydrogenating the isobutenethus-formed into isobutane. The cracking of a tertiary alkyl ether intoits corresponding alcohol and iso-olefin and the hydrogenation of aniso-olefin into its corresponding iso-alkane are well-known in the art.The cracking and hydrogenation may be carried out in any suitable wayknown in the art.

Alternatively and preferably, the alkyl tertiary butyl ether is directlyconverted into tertiary butane and the alcohol, i.e. in a single step.The cracking and hydrogenation is then combined by contacting the alkyltertiary butyl ether with a hydrocracking catalyst in the presence ofhydrogen. Any suitable hydrocracking catalyst may be used for this step.Such catalyst comprises a hydrogenating function, preferably ahydrogenating metal, supported on an acidic support material.Preferably, the catalyst comprises an acidic support material selectedfrom zeolitic or amorphous silica alumina and alumina. Amorphous silicaalumina is a particularly preferred support material. The hydrogenationfunction is preferably a hydrogenating metal selected from Group VIIImetals, more preferably selected from Pt, Pd, Ru, Rh, Ir, Ni andcombinations thereof. Hydrogenating metal that do not easily convertmethanol into carbon monoxide and hydrogen under the hydrocrackingconditions prevailing in this step are particularly preferred. Examplesof such hydrogenating metals are Pt and a combination of Pt and Ru.

Where the reaction product in step (a) comprises ethylene, at least partof the ethylene may be further converted into at least one ofpolyethylene, mono-ethylene-glycol, ethylbenzene and styrene monomer.Where the reaction product in step (a) comprises propylene, at leastpart of the propylene may be further converted into at least one ofpolypropylene and propylene oxide.

DETAILED DESCRIPTION OF THE DRAWING

In the FIGURE, an embodiment of the invention is schematically shownwherein the isobutene-depleted unsaturated C4 hydrocarbon stream andpart of the alkyl tertiary butyl ether formed in step d) are recycled tostep a). The alkyl tertiary butyl ether formed in step d) is recycledafter conversion into tertiary butanol and further conversion intoisobutene.

Methanol is fed via line 1 to oxygenate conversion reaction zone 10comprising an oxygenate conversion catalyst. In reaction zone 10,methanol is converted into olefins and water. Effluent from reactionzone 10 is supplied via line 11 to water quench tower 12 to be separatedinto water and an olefinic product stream. Water is withdrawn from tower12 via line 13 and the olefinic product stream is supplied via line 14to fractionation section 16. Fractionation section 16 comprises ade-ethaniser, a de-propaniser and a de-butaniser (not shown). Theolefinic product stream is first fractionated by means of thede-ethaniser and de-propaniser into an ethylene-rich stream, apropylene-rich stream, a C4+ hydrocarbon fraction and a lighter streamcomprising light by-products such as methane and carbon oxides. The C4+hydrocarbon fraction is further fractionated in the de-butaniser into aC4 hydrocarbon fraction comprising isobutene and a fraction rich in C5+hydrocarbons. The lighter stream, the ethylene-rich stream, thepropylene-rich stream and the fraction rich in C5+ hydrocarbons arewithdrawn from fractionation section 16 via lines 17, 18, 15 and 20,respectively. The fraction comprising C4 hydrocarbons is fed via line 21to first extractive distillation column 30. Solvent (NFM) is supplied tocolumn 30 via line 31. A stream 33 comprising solvent and butadiene iswithdrawn from the bottom of column 30, and a stream comprising butanesand butenes is withdrawn from the top of column 30 via line 32 andsupplied to second extractive distillation column 40. Traces ofbutadiene that might be present in the top stream of column 30 may beremoved by selective hydrogenation prior to supplying the top stream tosecond extractive distillation column 40. In column 40, the streamcomprising butanes and butenes is separated into a stream enriched inbutanes and a stream enriched in butenes. To column 40, NFM is suppliedas solvent via line 41. A stream enriched in butanes is withdrawn fromthe top of column 40 via line 42 and a stream comprising NFM and butenesis withdrawn from the bottom of column 40 via line 43. After removal ofthe NFM (not shown) a stream enriched in butenes is supplied toetherification reaction zone 50.

Methanol is supplied via line 51 to reaction zone 50 comprising anetherification catalyst. In etherification reaction zone 50, isobuteneis reacted with methanol to form methyl tert-butyl ether (MtBE). Theeffluent of reaction zone 50 is supplied via line 52 to separator 53 tobe separated into an isobutene depleted C4 hydrocarbon stream and anMtBE-enriched stream. The isobutene depleted C4 hydrocarbon stream isrecycled to reaction zone 10 via line 54. The MtBE-enriched stream iswithdrawn from separator 53 via line 56 and supplied to MtBEhydrocracking zone 60. Hydrogen is supplied to hydrocracking zone 60 vialine 61. In zone 60, MtBE is converted into methanol and isobutane.Methanol is recycled to etherification zone 50 via line 62. Part of themethanol may be recycled to oxygenate conversion zone 10 (recycle notshown). Isobutane obtained in zone 60 is supplied via line 63 tooxidation reaction zone 70. Air is supplied as oxidant to zone 70 vialine 71. In zone 70, isobutane is oxidised to tertiary butylhydroperoxide and tertiary butanol. The tertiary butyl hydroperoxideformed in zone 70 is supplied via line 72 to epoxidation zone 80. Thetertiary butanol formed is withdrawn via line 73. Part of thepropylene-rich stream separated in fractionation section 16 is suppliedto zone 80 via line 81. In zone 80, propylene oxide and tertiary butanolare formed. Propylene oxide is withdrawn as product via line 82. Thetertiary butanol formed is withdrawn via line 83 and, combined with thetertiary butanol in line 73, supplied to tertiary butanol dehydrationzone 90 and dehydrated into isobutene. Water is withdrawn from zone 90via line 91. Part of the isobutene thus-formed is recycled to oxygenateconversion zone 10 via line 92 and part of the isobutene is recycled toMtBE hydrocracking zone 60 via line 93.

The invention is illustrated by the following non-limiting example.

EXAMPLE

Model calculations were carried out for a process configuration as shownin the FIGURE, except that a single extractive distillation column isused (no prior removal of butadiene by extractive distillation).

A stream of 3349 kilotons per annum (kton/a) of methanol, 114 kton/a ofa recycle stream of isobutene and 240 kton/a of a recycle stream ofisobutene-depleted C4 hydrocarbons are supplied to oxygenate conversionzone 10. The isobutene-depleted C4 hydrocarbons stream comprises 177kton/a of normal butenes, 53 kton/a of C4 paraffins and 10 kta ofmethanol. Zone 10 contains a zeolitic catalyst comprising ZSM-23 andZSM-5 with a silica-to-almina ratio of 280 in a weight ratio of 4 to 1,a binder and matrix material. In zone 10, water and an olefinic productstream are formed. Fractionation yields 1146 kton/a of lower olefins, aC4 hydrocarbon fraction (363 kton/a) and a stream rich in C5+hydrocarbons. The C4 hydrocarbon fraction is fed to the extractivedistillation zone 40. In the extractive distillation zone, the C4hydrocarbon fraction is contacted with NFM as solvent in a solvent tofeed ratio of 5. A top product stream of 17 kton/a of C4 paraffins and 2kton/a of C4 n-butenes is obtained. The 17 kton/a of C4 paraffin thatare removed with the top stream from extractive distillation zone 40matches the amount of C4 paraffins produced in oxygenate-to-olefinsreaction zone 10. Hence, C4 paraffins are not built up in the recycle toreaction zone 10. A bottom product stream comprising solvent and 291kton/a of n-butenes, 53 kton/a of C4 paraffins is obtained in extractivedistillation zone 40. The NFM solvent is removed in a stripping zonethat is not shown in the FIGURE.

After stripping, a stream enriched in unsaturated C4 hydrocarbon isobtained that contains 291 kton/a of n-butenes and 53 kton/a of C4paraffins. This stream and 75 kton/a of methanol (65 kton/a recycledfrom MtBE hydrocracking zone 60 plus 10 kton/a make-up from an externalmethanol supply) is fed to etherification reaction zone 50 and MtBE isformed. After separation of the MtBE from the remaining C4 hydrocarbons,an azeotropic mixture of 230 kton/a of isobutene-depleted C4hydrocarbons and 10 kton/a methanol is obtained and recycled tooxygenate-to-olefins reaction zone 10.

A stream of 179 kton/a of MtBE separated from the effluent ofetherification reaction zone 50, 369 kton/a of isobutene from isobutanoldehydration zone 90, and 17 kton/a of hydrogen are fed to MtBEhydrocracking zone 60 to form 500 kton/a of isobutane and 65 kton/a ofmethanol. The methanol is recycled to etherification reaction zone 50.The isobutane is supplied, together with air, to oxidation reaction zone70 to form tert-butyl hydroperoxide and tertiary butanol. The tert-butylhydroperoxide and 181 kton/a of the propylene produced in zone 10 areconverted into 250 kton/a of propylene oxide in epoxidation zone 80. Thetertiary butanol formed in zones 70 and 80 is dehydrated in dehydrationzone 90 to form 155 kton/a of water, and 483 kton/a of isobutene. Partof the isobutene (114 kton/a) is recycled to oxygenate conversion zone10 and part (369 kton/a) is recycled to MtBE hydrocracking zone 60.

TABLE Product streams in kilotons per annum in the EXAMPLE line CompoundEXAMPLE 1 methanol 3349 42 Normal-butane and isobutane 17 54isobutene-depleted C4 hydrocarbons 240 including MeOH (azeotropicmixture) 61 hydrogen 17 63 isobutane 500 81 propylene 181 82 propyleneoxide 250 92 isobutene 114 93 isobutene 369

What is claimed is:
 1. A process for preparing lower olefins from an oxygenate, the process comprising the following steps: a) contacting the oxygenate with a molecular sieve-comprising catalyst, at a temperature in the range of from 350to 1000° C. to obtain an olefinic product stream comprising ethylene, propylene and C4 hydrocarbons; b) separating ethylene and/or propylene and a fraction comprising C4 hydrocarbons including saturated and unsaturated C4 hydrocarbons, from the olefinic product stream; c) subjecting at least part of the fraction comprising C4 hydrocarbons to extractive distillation to obtain a stream enriched in unsaturated C4 hydrocarbons comprising isobutene and n-butenes, and a stream enriched in saturated C4 hydrocarbons; d) supplying at least part of the stream enriched in unsaturated C4 hydrocarbons obtained in step c) and an alcohol selected from the group consisting of methanol, ethanol and a mixture thereof, to an etherification reaction zone comprising an etherification catalyst and reacting, in the etherification reaction zone, at least part of the isobutene in the stream enriched in unsaturated C4 hydrocarbons with the alcohol to obtain an etherification product stream comprising alkyl tertiary butyl ether; e) separating the etherification product stream into an alkyl tertiary butyl ether-enriched stream and an isobutene-depleted unsaturated C4 hydrocarbon stream; and f) recycling at least part of the isobutene-depleted unsaturated C4 hydrocarbon stream to step a).
 2. A process according to claim 1, wherein at least part of the alkyl tertiary-butyl ether stream is recycled to step a).
 3. A process according to claim 1, where the oxygenate is methanol, dimethylether, or a mixture thereof.
 4. A process according to claim 1 wherein the oxygenate in step (a) is an alcohol and the same alcohol is used as alcohol in etherification step (d).
 5. A process according to claim 1 wherein the alkyl tertiary butyl ether in the alkyl tertiary butyl ether-enriched stream is decomposed to at least an alcohol and the alcohol is recycled to step (d).
 6. A process according to claim 1 further comprising: g) converting at least part of the alkyl tertiary butyl ether in the alkyl tertiary butyl ether-enriched stream into the alcohol and isobutane; h) oxidizing isobutane obtained in step g) into tertiary butyl hydroperoxide; and i) reacting tertiary butyl hydroperoxide obtained in step h) with ethylene and/or propylene separated from the olefinic product stream obtained in step a) to obtain an epoxide and tertiary butanol.
 7. A process according to claim 6, wherein in step g) the alkyl tertiary butyl ether is first converted into isobutene and the alcohol and the isobutene is then hydrogenated into isobutane.
 8. A process according to claim 6, wherein in step g) the alkyl tertiary butyl ether is directly converted into isobutane and the alcohol by contacting alkyl tertiary butyl ether with a hydrocracking catalyst in the presence of hydrogen.
 9. A process according to claim 6 wherein at least part of the tertiary butanol obtained in step i) is recycled to oxygenate conversion step a).
 10. A process according to claim 1 wherein the molecular sieve-containing catalyst is a zeolite-comprising catalyst.
 11. A process according to claim 10, wherein the zeolite-comprising catalyst comprises at least one zeolite selected from MFI, MEL, TON and MTT zeolites.
 12. A process according to claim 11, wherein the zeolite-comprising catalyst comprises at least one of ZSM-5, ZSM-11, ZSM-22 and ZSM-23 zeolites.
 13. A method according to claim 1 wherein the reaction product in step (a) comprises ethylene and at least part of the ethylene is further converted into at least one of polyethylene, mono-ethylene-glycol, ethylbenzene and styrene monomer.
 14. A method according to claim 1 wherein the reaction product in step (a) comprises propylene and at least part of the propylene is further converted into at least one of polypropylene and propylene oxide.
 15. A method according to claim 1 wherein at least part of the stream enriched in unsaturated C4 hydrocarbons is used in at least one of a metathesis with ethylene to produce additional propylene, an alkylation process to produce alkylate and polymerization of ethylene to produce polyethylene. 